Disc drives are typically used as primary data storage devices in modern computer systems and networks, due to the efficient and cost-effective manner in which large amounts of computerized data can be stored and retrieved. Disc drives of the present generation have data storage capacities measured in excess of several gigabytes (GB) and can be used alone (as in a typical personal computer configuration) or in multi-drive data storage arrays (as with an internet network server or a mainframe computer).
A typical disc drive comprises a plurality of rigid magnetic storage discs which are axially aligned and arranged about a spindle motor for rotation at a constant high speed (such as around 10,000 revolutions per minute). An array of read/write heads are provided to transfer data between tracks of the discs and a host computer in which the disc drive is mounted.
The heads are mounted to a rotary actuator assembly and are controllably positioned adjacent the tracks by a closed loop servo system. More particularly, the actuator assembly includes a coil of a voice coil motor (VCM), so that the servo system controls the position of the heads through the application of current to the coil in response to detected and estimated positions of the heads, as well as command inputs indicating desired positions of the heads.
The servo system primarily operates in one of two selectable modes: seeking and track following. A seek operation entails moving a selected head from an initial track to a destination track on the associated disc surface through the initial acceleration and subsequent deceleration of the head away from the initial track and toward the destination track. A velocity control approach is used whereby the velocity of the head is repeatedly estimated (based on measured position) and compared to a velocity profile defining a desired velocity trajectory for the seek. Corrections to the amount of current applied to the coil during the seek are made in relation to the difference between the estimated velocity and the desired velocity.
At such time that the head reaches a predetermined distance away from the destination track (such as one track away), the servo system transitions to a settling mode wherein the head is settled onto the destination track. Thereafter, the servo system enters a track following mode of operation wherein the head is caused to follow the destination track until the next seek operation is performed.
Disc drive designs thus typically use proximate time optimal control with a velocity profile to control a selected head during a seek, a state estimator based controller with relatively slow integration to settle the head onto the destination track, and the same state estimator based controller with relatively fast integration for track following.
One problem with this approach is that, at such time that control is switched from seek mode to settling mode, an initial velocity variation can cause large overshoot or undershoot of the head relative to the destination track, undesirably extending the time required to settle the head onto the destination track. This velocity variation is inherent in modern disc drive designs because the acceleration constant of the VCM changes with temperature and relative position of the coil. These and other such factors tend to introduce velocity errors during seeks, undesirably extending settling times.
A typical high performance disc drive has a specified seek time of around eight milliseconds (msec), which includes the time necessary for the head to be moved across the surface of the disc and settled onto the destination track (over the track center within a specified tolerance), as well as the latency time required for the desired data sector to reach the head as the disc rotates relative to the head. Significantly, such velocity variations at the beginning of the settling phase can extend the seek time by one to two milliseconds. Because the disc drive is typically unavailable to transfer data between the discs and a host computer during a seek operation, consumer demands for continually improved data transfer performance has led designers to find ways to improve servo control and reduce settling time during seeks.
Several methods have been recently proposed to reduce settling time using a technique generally referred to as initial value compensation. See, for example, an article by Eddy et al. entitled "Bias in Disk Drive Rotary Actuators: Characterization, Prediction and Compensation," IEEE Transactions on Industrial Electronics, Vol. 33, No. 3, 1997, pp. 2424-2433, which describes a backward search methodology for proper controller initialization. However, this approach places relatively large real-time computational demands upon the system, limiting its usefulness in real world disc drive applications where processing resources are usually limited.
Three additional methodologies were proposed by Yamaguchi et al. in "Design of Mode Switching Controller with Initial Value Compensation and its Application to Disk Drive Servo Control," IFAC 13.sup.th Triennial World Congress, San Francisco, Calif., USA, 1996, pp. 471-476, and "Design of Mode Switching Controller with Initial Value Compensation and its Application to Head Positioning Control on Magnetic Disk Drives," IEEE Transactions on Industrial Electronics, Vol. 43, No. 1, 1996, pp. 65-73. The first two methodologies proposed by Yamaguchi et al. in the foregoing papers involves resetting controller (observer) states at settling mode initialization. Although operative, such approaches, at least to some extent, impede other critical tasks of the disc drive servo system, such as setting on-track check criterion, adaptively calibrating gain, etc. The third methodology proposed by Yamaguchi et al. involves canceling all closed-loop poles of the track following controller and replacing these poles at desired positions. However, like the methodology proposed by Eddy et al., this approach can be undesirably complex, in that an additional high order compensator is necessary for the initial value of the states. Also, there is generally no easy way to adapt the system to account for changes in temperature and similar factors which tend to adversely affect servo performance.
Accordingly, there is a continual need in the art for improvements whereby settling characteristics of a disc drive can be adaptively optimized in the presence of parametric variations which tend to introduce significant levels of head velocity errors.